The murine teratocarcinoma system has become an important model of neoplastic differentiation. The undifferentiated stem cells, termed embryonal carcinoma (EC), are highly malignant, but the differentiated progeny are, with rare exception, benign. The EC stem cells can be induced to differentiate under the influence of several chemical agents, the most potent being retinoic acid (RA). Embryonal carcinoma tumor differentiation can be induced in vivo with a combination of agents resulting in prolonged survival of tumor-bearing hosts and, in some cases, cure of the malignancy by complete conversion to benign cells. The process can be studied in vitro, but at present little is known about mechanisms of action of inducing agents due, in part, to a lack of a suitable cell system and methods to quantitate differentiation. We will develop a rapid, reproducible method of quantitating EC differentiation and of analyzing differentiation quantitatively in an EC cell system highly sensitive to chemical induction. Recently, we have discovered that ouabain induces differentiation in murine EC cells. Since the pharmacological action of ouabain is well established (inhibition of Na+-K+ ATPase), this may provide a clue to the mechanism of action of some of the other chemical agents known to induce EC differentiation. In separate but related studies, we have established a new human EC cell line. This cell line undergoes spontaneous differentiation to epithelial and giant cells. Spontaneous differentiation is minimized by growth of EC gells as aggregates on irradiated feeder layers. In this setting, dimethyl acetamide, but not retinoic acid, can induce significant differentiation. These studies could provide a necessary link from the murine model system to the application of differentiation-induction therapy for clinical, human teratocarcinomas. (M)